Despite decades of research, the physics of lightning leader propagation remains poorly understood. Observations have shown that lightning's initial path to the ground is forged by a hot leader that breaks down the virgin and electrically insulating air in front of it and thus allows electrical current to flow in the channel behind the front. This process can continue for great distances, permitting the initially localized breakdown inside the thundercloud to traverse many kilometers of air to the ground where it can potentially cause injury to people and damage to property.
The initial leader is known to not travel to the earth in a continuous manner, but, instead, takes a series of discrete steps. For this reason, the initial leader in natural cloud-to-ground lightning is referred to as the stepped-leader. Why or how lightning propagates in this halting manner is not known, but one possible scenario based upon laboratory spark experiments is that during the stepping process, a new leader channel segment is initiated in a high electric field region some distance in front of the stalled leader. This new segment attaches to the old leader channel, allowing current to rush along the channel and extending its overall length. The leader propagation then stalls and the whole cycle repeats. Because the process of stepping determines the path that lightning takes and the number of forks and branches that develop, understanding how lightning steps occur is of great practical interest. It has applications for the wider fields of atmospheric science, gas discharge physics, plasma physics, and planetary sciences, as well as lightning safety and protection devices for both humans, animals and electronic and utility power equipment. Additionally, it is of significant interest to subsidiary fields such as the insurance industry and electric power utilities to ascertain whether lightning has struck an object or structure.
Up until recently most researchers believed that lightning was an entirely conventional, albeit large, discharge that did not involve any high-energy processes that might produce energetic radiation. This view was challenged in 2001 when Moore et al. published a paper entitled “Energetic radiation associated with lightning stepped-leaders” (Geophys. Res. Lett. 28, 2141-2144, 2001; hereafter “Moore”) which reported that “energetic radiation” which could be electrons or high energy photons (gamma rays or x-rays), is produced during natural lightning. Moore did not report regarding the energy spectrum of the disclosed energetic radiation nor any temporal characteristics other than the start time of the energetic radiation relative to the time of the ground return stroke. In addition, Moore did not provide reliable estimates of the energetic radiation source location, and could not rule out more remote sources of the energetic radiation which would prevent a determination of the strike location based upon the energetic radiation measurements. Moreover, Moore did not estimate the intensity of the energetic radiation at the source nor the distance that it propagates through the air, since such information requires knowledge of the type of energetic radiation and its energy spectrum.
Presently about 85 to 90% of lightning flashes can be located with an average accuracy of 500 meters (the remaining percentage not being detected at all) by the U.S. National Lightning Detection Network (NLDN) and similar networks in other countries. The strike location is identified by timing and direction finding on the lightning RF (about 10 kHz to about 300 kHz) signal. A method and system which provides better localization, such as to 100 meters or better, would be highly desirable to substantially more accurately determine whether a given location was in fact struck by lightning. Such a determination can be important particularly regarding insurance claim related issues.